The present invention relates to improvements in papermaking machines, and more specifically to a twin wire former where the wires are guided into the forming throat by turning bars and the cross machine curvature of the turning bars is controlled.
A typical paper web forming section includes a headbox for ejecting paper stock onto a moving flat screen known as a fourdrinier wire. The stock from the headbox impinges against the moving fourdrinier wire and water within the stock is drained downwardly through the wire so that a fibrous web is formed on the upper surface of the fourdrinier wire.
However, due to the downward drainage of water from the web, the resultant web has an upper surface which displays different surface characteristics from the lower surface of the web. Such two-sidedness, or lack of uniformity between the upper and lower surfaces of the web, has caused problems in the finished product when used for printing in which uniformity of the surfaces is desirable.
Accordingly, forming machines were developed which enabled the web to be dewatered upwardly as well as downwardly. These so-called "twin-wire" machines include an upper wire loop which cooperates with the lower fourdrinier wire such that the stock entering a forming section defined by the upper and lower wires is typically subjected to vacuum applied through the upper wire for removing a further portion of water from the web upwardly through the second wire.
The twin-wire concept was modified by moving the headbox to adjacent a first end of the forming section defined by the cooperating twin wires. The resultant forming section is known as the "BEL BAIE FORMER". BEL BAIE is a registered trademark of Beloit Corporation. The BEL BAIE forming section includes a pair of breast rolls for guiding the respective wires so that the wires cooperate together to define the aforementioned forming section. However, of necessity, the rotating breast rolls are of a diameter such that it is difficult to position the slice lip of the headbox very close to the first end of the forming section.
Experiments have indicated that when the slice lip of the headbox is moved nearer to the first end of the forming section, less disturbance and breaking up of the stock occurs between the headbox and the first end and the uniformity of the resultant web is improved. Furthermore, less streaking occurs when the slice lip is brought up very close to the first end as the angle at which the stock impinges, or first contacts the lower wire is minimized.
An important development in the advance of dewatering by twin wire machines includes reducing the distance between the headbox and the first end of the forming section by using stationary turning bars in place of rotatable breast rolls. In order to accurately guide the path of the forming wires as they enter the throat, the turning bars must be accurately positioned and this is particularly true for very wide machines. Temperature differences due to friction on the turning bars and due to the temperature of the stock and ambient factors frequently affect the curvature of the turning bars so that the travel of the wire over the bars is varied thus causing turbulence in the stock and unequal initial dewatering. In some cases it may be desirable for the turning bars to remain absolutely straight across the width of the machine, or in other cases the turning bars may be positioned so that they are formed with a slight crown for insuring the spread of the wire as it enters the throat. Also, because of other effects on the turning bars, it may be desired to apply a force of the turning bars to cause an effective force which tends to cause a negative crown on the bars. In any event in addition to the other attendant advantages of using turning bars instead of breast rolls, the ability to control the deflection of the turning bars is a marked advantage over breast rolls which must be provided with a predetermined fixed crown or fixed shape in a cross machine direction. Because of the limited space available in the machine direction, the application of forces to the turning bars to control their cross machine crown is very difficult.
It is accordingly an object of the present invention to provide an improved support for the wires in a twin wire forming machine where the wires are entering the throat of the former.
A further object of the invention is to provide turning bars for supporting the wires of a twin wire forming machine wherein the wires enter the throat over turning bars and the deflection or curvature of the turning bars in a cross machine direction is controlled to obtain a predetermined crown or shape of the bar in a cross machine direction.
A further object of the invention is to provide an improved mechanism for controlling the shape or curvature of a turning bar supporting a traveling forming wire in a paper machine utilizing the expansion and contraction effects of thermally heated or cooled water on metal parts.